Fermentation processes for producing organic acids, such as acetic and lactic acids, go through an intermediate production of salts, such as ammonium acetate or lactate. Hence, salts are the byproducts or intermediate products of a number of chemical processes. For example, regenerable flue gas desulfurization processes use a sodium alkali to absorb the SO.sub.2, thus resulting in a soluble bisulfite salt, NaHSO.sub.3. Production of soda ash (Na.sub.2 CO.sub.3) requires the processing of the raw material salt viz., trona(Na.sub.2 CO.sub.3.NaHCO.sub.3.2H.sub.2 O) or the naturally occurring brines. In magnetohydrodynamic power generation process the potassium carbonate seed material absorbs SO.sub.2 in the fuel and is converted to a byproduct potassium sulfate.
Electrodialysis(ED) may be used to convert these and other soluble salts directly into their acid and base components. For example, such a procedure enables a direct recovery of a relatively pure form of the organic acid from its organic salt. The co-product base (ammonia for example) may be recovered for reuse in the fermentation process for pH adjustment, thus permitting an economical and environmentally superior option for producing organic acids. In other instances, such as with sodium bisulfite, trona or potassium sulfate, the electrodialysis offers an environmentally superior route for recovering or recycling the acid, base components.
Electrodialysis uses direct current as a means for causing a movement of ions in the solutions of the processing streams of salt starting material. Electrodialysis processes are usually carried out in an arrangement comprising a stack where a plurality of flat sheet ion exchange membranes and gasket sheets are clamped together. These sheets provide flow paths for containing salt materials that produce acids and bases. The process unit requires a means for splitting water into hydrogen (H.sup.+) and hydroxyl (OH.sup.-) ions.
Two useful means for splitting water are:
(1) A bipolar membrane or a bipolar module formed by a combination of cation and anion membranes which functions as a bipolar membrane. Suitable bipolar membranes are available from Aqualytics, a division of Graver Water, and from Tokuyama Soda, and from the Formic Corporation; and PA1 (2) An electrode set comprising an anode and a cathode. The electrodes, particularly the anodes, are coated for chemical stability, for minimizing power consumption, and for the formation of byproducts other than hydrogen (at cathode) and oxygen (at the anode), among other things. Suitable electrodes are available from the Eltech Corporation, the Electrode Products Inc., and from others. A hydrogen depolarized anode can also generate H.sup.+ ions in an aqueous solution of an electrode stream next to the anode.
As described in my above-identified co-pending applications, the stack contains electrodes (anode and cathode) at either end and a series of membranes and gaskets which have open active areas in their middle to form a multiplicity of compartments which are separated by the membranes. Usually, a separate solution (an electrode stream) is also supplied to each of the compartments containing the electrodes. Special membranes may be placed next to the electrodes to prevent a mixing of the process streams with the electrode streams.
The majority of the stack between the electrode compartments comprises a repeating series of units of different membranes with solution compartments between adjacent membranes. Each of the repeating units is called the "unit cell" or simply a "cell." The solution is supplied to the compartments by internal manifolds formed as part of the gaskets and membranes or by a combination of internal and external manifolds. The stacks can include more than one type of unit cell.
Streams of processing fluids may be fed from one stack to another in order to optimize process efficiency. After one pass through the stack, if the change in the composition of a process stream is relatively small, the process solutions can be recycled by being pumped to and from recycle tanks. An addition of fresh process solution to and withdrawal of product from the recycle loop can be made either continuously or periodically in order to maintain the concentration of products within a desired range.
When bipolar membranes are used to form acid or base from the salt, in order for the membrane to function as a water splitter, the component ion exchange layers must be arranged so that the anion selective layer of each membrane is closer to the anode than the cation selective layer. A direct current passed through the membranes in this configuration causes water splitting with OH.sup.- ions being produced on the anode side and a corresponding number of H.sup.+ ions being produced on the cathode side of the membranes. The dissociated salt anions move toward the anode. The dissociated salt cations move toward the cathode.
The electrolysis process works in a similar manner, with the water splitting occurring at the two electrodes. When a direct current appears, water molecules are converted to oxygen gas at the anode along with the introduction of H.sup.+ ions into the aqueous solution. At the cathode, the water molecules are converted to hydrogen gas along with the introduction of OH.sup.- ions into the aqueous solution. In the hydrogen depolarized anode based electrolysis unit, OH.sup.- ions are released into the aqueous solution next to the cathode. While released, the hydrogen gas is forwarded to the catalytic hydrogen depolarized anode for H.sup.+ ion generation.
Electrodialysis equipment for acid/base production may have three compartment cells comprising bipolar, cation and anion membranes; two compartment cells containing bipolar and cation (or anion) membranes; multichamber two compartment electrodialysis cells comprising bipolar and two or more cation membranes. The term "bipolar membrane" also includes bipolar equivalent structures, such as the use of electrodes and composite bipolars. FIG. 1 shows the unit cell for the three most useful configurations.
Specific references are:
"Electrodialysis Water Splitting Technology" by K. N. Mani; J. Membrane Sci., (1991), 58, 117-138 PA0 U.S. Pat. Nos. 4,082,835; 4,107,015; 4,592,817; 4,636,289; 4,584,077; 4,390,402; and 4,536,269.
In accordance with an aspect of this invention, an electrodialysis apparatus is improved through the addition of a nanofiltration unit upstream of an electrodialysis cell or through the use of a downstream ion exchange column in combination with the electrodialysis cell and in communication with the base loop of the cell. Or both nanofiltration and an ion exchange column may be used.
In another aspect of the invention, the use of monovalent selective cation membranes in combination with adequate dilution of the base product allows reliable non-fouling operation of the electrodialysis cell to produce acid and base from salt.